This invention relates to high impact monovinylidene aromatic polymer compositions. High impact polystyrene (HIPS) compositions comprising polystyrene having a particulate elastomeric material dispersed therein are well known in the art as are uses for such materials. It is also known that the physical properties of these types of compositions are affected by the relative amounts of elastomer particles having different sizes and different particle structures. With regard to the size of the particles, it is known that the larger rubber particles having diameters greater than about 0.5 microns (.mu.) contribute greatly to impact resistance but tend to reduce the gloss of articles formed or molded from compositions containing those particles. This is especially true in the case of the mass-type of particles which will be described below. On the other hand, when smaller modifying rubber particles are used in polymer compositions, articles formed therefrom ten to be glossier but are less impact resistant than if the same amount of rubber was used in the form of larger particles.
Concerning the structures of the individual rubber particles (i.e., rubber particle morphology), there are two general types of rubber particles which can be used in rubber-modified polymer compositions. One type of particle is the occlusion containing mass particles, the other is the generally solid emulsion particles. In both cases, the rubber particles are usually grafted with amounts of a polymer compositionally similar, preferably identical, to the matrix polymer in which the particles are to be dispersed. It is generally believed that the grafted rubber particles containing occlusions of matrix polymer therein provide more impact resistance than the same amount of rubber in the form of similarly grafted emulsion particles. The grafted occlusion-containing rubber particles are formed and grafted in the well-known mass, mass-solution or mass suspension types of polymerization process where a previously produced rubber is dissolved in an amount of polymerizable monomer or in a mixture or solution of polymerizable monomer(s) with optional diluents, which monomer(s) are thereafter polymerized. The polymerization of the monomers with agitation causes the formation of occlusion-containing particles of rubber by the well known process of phase inversion. In a mass-suspension process, the reaction mass is then suspended in water and polymerization completed. Particles produced in such mass, mass-solution or mass-suspension processes, or variations of these processes, containing occlusions of the grafted polymer are hereinafter reference to a "mass particles".
The other main type of rubber particle morphology, the emulsion rubber particle, is achieved by emulsion polymerization of the rubber in an aqueous latex, forming a group of rubber particles. After the rubber particles are made, polymerizable and graftable monomers (e.g., styrene) are usually added to the rubber containing latex and polymerized to form the graft portion of these particles and, optionally, amounts of matrix polymer. This type of rubber particle, produced via an emulsion polymerization process is hereinafter referred to as an "emulsion particle". Due to their small size, emulsion particles are generally able to provide a limited degree of impact improvement while maintaining the gloss of articles molded from compositions which contain them. It is also known that small rubber particles can be agglomerated to form larger, agglomerate particles, which larger agglomerate particles can be used to improve the impact resistance of polymeric compositions while reducing gloss slightly.
In view of these phenomena, a great deal of effort has gone into achieving optimized property distributions by tailoring the rubber particle distributions (i.e., the relative amounts of the rubber particles of particular sizes and structures), see for example, Canadian Patent No. 832,523; and U.S. Pat. Nos. 3,592,878, 3,825,621, 4,012,462, 4,146,589, 4,153,645, 4,214,056, 4,221,883, 4,334,039, and also European Patent Appln. No. 48,389 which are incorporated herein by reference. In these references, high impact polystyrene compositions having "bimodal" particle size distributions are taught. As used in the art and herein, a composition having a bimodal particle size distribution contains two distinct groups of rubber particles, each group having a different average particle size and/or having a different particle structure.
In many of the utilizations of polymeric compositions, they are exposed to both organic solvents and stress, such as when the compositions are molded into containers for oil- or fat-containing materials. The failure of materials when exposed to both solvent action and stress has become known as environmental stress cracking. As is known, the HIPS compositions of the prior art generally have poor environmental stress crack resistance in these types of situations and therefore either perform poorly when used in such situations or are not used at all.